In the cryopreparation of biological materials for microscopic, especially electron microscopic examinations, preparation chambers with a volume of more than 11 are being used with increasing frequency; in these chambers both the specimens and the necessary tools may be kept reproducibly at very low temperatures without icing up due to the condensation of atmospheric moisture. An important aid employed in the cryofixation of biological materials is a highly polished metallic surface ("metallic mirror") which is cooled to temperatures of less than 100.degree. K. by means of a suitable cryogen and on which the biological object of interest is placed by means of a precisely controlled, high-speed injector. Immediately after the object surface strikes the metallic mirror, a marginal area of the object about 10 to 20 um wide is frozen within an extremely short time by direct thermal contact with the metallic mirror at a cooling rate of more than 1,000.degree. C./second. In contrast to the freezing of the adjacent, deeper zones of the specimen, those which are more than 10 to 20 um removed from the marginal and "metal/object" contact zone, the marginal zone freezes rapidly (cryofixation by means of thermal shock); there is no separation detectable with the electron microscope of the aqueous, plasmatic, mixed phases which form the matrix of biological materials, whereas the deeper regions, due to the lower cooling rates of less than 1,000.degree. C./second occurring in them, exhibit separation phenomena which are characterized by the occurrence of ice crystals inside the cells and in the intercellular spaces. Since these ice crystals, as artificial products ("artefacts") rule out any meaningful examination of the microstructure or ultrastructure, one strives to keep the temperature difference between the normal temperature of the living object (e.g., 37.degree. C. in warm-blooded animals) and the freezing surface of the metallic mirror as large as possible in order to preserve the deepest possible marginal area of the specimen in a true-to-life state, i.e., without ice crystal artefacts, by means of the steepest possible temperature gradient. Thus, it is an obvious step to cool the metallic mirror by means of a cryogen whose temperature lies below the boiling temperature of liquid N.sub.2 (-196.degree. C.). To achieve this result either partially solidified nitrogen ("nitrogen slush", which has the melting temperature of .vertline.solid.vertline. nitrogen, -210.degree. C.) or liquid helium (minimum temperature at the condensation point: about 4.degree. K.) may be used.
A device for metallic mirror-cryofixation is known in which a freezing chamber having a volume of less than 100 ml is used; its floor is formed by the highly polished upper surface of a massive block of silver. The silver block is immersed in nitrogen slush which is contained in a Dewar vessel and reaches a temperature of less than -200.degree. C. In order to prevent condensation of atmospheric nitrogen on the metallic mirror, this freezing chamber may be flushed with circulating helium gas which is passed through the nitrogen slush in the Dewar before entering the freezing chamber and is therefore pre-chilled to a temperature which corresponds approximately to that of the silver mirror (see Journal of Microscopy, Vol. 111, Part 1, pp. 35-38, September, 1977). Because of the small volume of its freezing chamber, cryopreparation is not feasible with this known device.